INCOME TAXES - Additional Information (Details) - USD ($)		3 Months Ended		12 Months Ended
	Dec. 27, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Income Taxes [Line Items]
Effective income tax rate expense, percent		16.00%	3.80%
Income tax expense (benefit)		$ 5,999,000	$ (216,000)
Long-term asset		75,765,000		$ 64,526,000
Korean Government [Member]
Income Taxes [Line Items]
Estimated settlement amounts due (refunds) from withholding taxes				(36,900,000)
U.S.Federal Government [Member]
Income Taxes [Line Items]
Estimated settlement amounts due (refunds) from withholding taxes				16,200,000
KNTS [Member]
Income Taxes [Line Items]
Income tax withholding and interest		13,200,000
Reduction of Korean Withholding Taxes [Member]
Income Taxes [Line Items]
Reduction of deferred tax asset for foreign tax credits and R&D				$ 20,700,000
Allocation of Withholding to Non-Korean Patents [Member]
Income Taxes [Line Items]
Long-term asset	$ 17,000,000
Allocation of Withholding on Korean Patents [Member]
Income Taxes [Line Items]
Income tax expense (benefit)	$ 1,600,000
ASU No. 2016-09 [Member]
Income Taxes [Line Items]
Income tax expense (benefit)		$ (1,500,000)	$ (1,300,000)